The invention relates to a circuit arrangement for operating a load comprising a DC-AC-converter equipped with
a first input terminal and a second input terminal for connection to the poles of a supply voltage source,
a first series arrangement comprising a first switching element and a second switching element and coupled between the input terminals,
a second series arrangement comprising a first capacitive element and a second capacitive element and shunting the first series arrangement,
a load circuit connecting a first terminal between the first and second switching element and a second terminal between the first capacitive element and the second capacitive element and comprising connection terminals for connection to the load,
control means for rendering the first and second switching element alternately conducting and non-conducting, coupled between the load circuit and control electrodes of the first and the second switching element,
a start circuit comprising
a third series arrangement comprising a third capacitive element and a breakdown element and coupled between the control electrode and one of the main electrodes of the first switching element,
a fourth series arrangement comprising two impedances coupled to a third terminal between the third capacitive element and the breakdown element.
Such a circuit arrangement is known from EP 0 294 878 A1. In the known circuit arrangement the impedances comprised in the fourth series arrangement are ohmic resistors. The fourth series arrangement connects the first and the second input terminal and a terminal between the two ohmic resistors is connected to the third terminal. The load that is operated is a lamp. When the known circuit arrangement starts operating, the third capacitive element is charged by a current that flows from the first input terminal through one of the ohmic resistors, the load circuit and the second capacitive element to the second input terminal. When the voltage over the third capacitive element has reached a value that is approximately equal to the breakdown voltage of the breakdown element, the breakdown element becomes conductive and the third capacitive element is discharged through the breakdown element and the control electrode and a main electrode of the first switching element. The first switching element is thereby rendered conductive. In the known circuit arrangement the first switching element is the switching element that during operation is connected to the positive pole of the supply voltage source. This is advantageous because less components are needed in the start circuit in case this first switching element is rendered conductive first by the start circuit than are needed in case the second switching element is rendered conductive first. Thus the oscillation of the circuit arrangement is started by means of a start circuit that comprises only a limited amount of components and functions in a very dependable way. A disadvantage of the known circuit arrangement, however, is the fact that a DC voltage can be present over the third capacitive element during stationary operation. The presence of such a DC voltage can be caused by a difference between the conduction times of the first and the second switching element, more in particular when the duty cycles of the switching elements are below 40%. This difference between the conduction times is caused in turn by differences between the electrical properties of the switching elements. The DC-voltage over the third capacitive element influences the switching of the first switching element during stationary operation. More in particular there is the risk that both switching elements become conductive at the same time causing a shortcircuit of the supply voltage and the risk that the circuit will shortly stop oscillating causing instabilities.
The invention aims to provide a circuit arrangement for supplying a load comprising a relatively simple and dependable start circuit that does not disturb the functioning of the circuit arrangement during stationary operation.
A circuit arrangement as mentioned in the opening paragraph is therefor characterized in that the fourth series arrangement is coupled between the first input terminal and the third terminal and in that a fourth terminal between the two impedances is connected to the second terminal.
When a circuit arrangement according to the invention starts operating, the third capacitive element is charged by a current that flows from the first input terminal, through the fourth series arrangement, the third capacitive element, the load circuit and the second capacitive element to the second input terminal. When the voltage over the third capacitive element has reached a value that is substantially equal to the breakdown voltage of the breakdown element, the breakdown element becomes conductive and the third capacitive element is discharged through the breakdown element and the control electrode and a main electrode of the first switching element. The first switching element is thereby rendered conductive, and the circuit arrangement starts oscillating. Thus the start circuit in a circuit arrangement according to the invention starts the oscillating of the circuit arrangement in an effective way although the start circuit comprises only a relatively small amount of components. During stationary operation the switching elements are rendered conductive and non-conductive alternately. As a result a substantially square wave voltage is present at the first terminal. At the second terminal a DC-voltage is present with an amplitude that approximately equals the average value of the substantially square wave voltage present at the first terminal. The first terminal is connected to the second terminal by means of a series arrangement of the third capacitive element and one of the impedances. The average value of the voltage present over this series arrangement is equal to zero. Therefore the voltage over the third capacitive element is also substantially equal to zero. As a consequence the switching of the switching elements during stationary operation is not adversely affected by the start circuit.
Good results have been obtained with embodiments of a circuit arrangement according to the invention, wherein the two impedances comprised in the fourth series arrangement are ohmic resistors.
Good results have also been obtained for embodiments of a circuit arrangement according to the invention in which the first and second switching elements are bipolar transistors.
In many embodiments of a circuit arrangement according to the invention part of the current path of the current charging the third capacitive element can flow through components that fulfil another function during stationary operation. In these embodiments it is not necessary to equip the circuit arrangement with further components to make sure that the current path of the current charging the third capacitive element is complete. In other embodiments of a circuit arrangement according to the invention, however, the current path for charging the third capacitive element is either incomplete or has a total impedance that is very high. This problem can be overcome by shunting the second switching element by means of a third impedance, preferably comprising an ohmic resistor.